Inchemical nomenclature, adescriptor is a notational prefix placed before thesystematic substance name, which describes the configuration or thestereochemistry of themolecule.[1] Some of the listed descriptors should not be used inpublications, as they no longer accurately correspond with therecommendations of theIUPAC. Stereodescriptors are often used in combination withlocants to clearly identify achemical structure unambiguously.
The descriptors, usually placed at the beginning of the systematic name, are not taken into account in the alphabetical sorting.
The descriptorscis (Latin,on this side of)[2] andtrans (Latin,over, beyond)[3] are used in various contexts for the description of chemical configurations:[4][5]
Inorganic structural chemistry, the configuration of adouble bond can be described withcis andtrans, in case it has a simple substitution pattern with only two residues. The position of two residues relative to one another at different points in a ring system or a larger molecule can also be described withcis andtrans if the structure's configuration is rigid and does not allow simple inversion.
In inorganiccomplex chemistry, the descriptorscis andtrans are used to characterize the positional isomers inoctahedral complexes with A2B4X configuration orsquare planar complexes with A2B2X configuration.
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The typographic presentation ofcis andtrans is italicised and in lower case letters.
Thecis/trans nomenclature is not unambiguous for more highly substituted double bonds and is nowadays largely replaced by the (E)/(Z) nomenclature.[6]
See:E-Z notation
The descriptors (E) (from Germanentgegen, 'opposite') and (Z) (from Germanzusammen, 'together') are used to provide a distinct description of the substitution pattern foralkenes,cumulenes or other double bond systems such asoximes.[7]
For the attribution of (E) or (Z) is based on the relative position of the two substituents of highest priority are on each side of the double bond, while the priority is based on theCahn–Ingold–Prelog (CIP) nomenclature. The (E)/(Z) nomenclature can be applied to any double bond systems (includingheteroatoms), but not tosubstitutedring systems. The descriptors (E) and (Z) are always capitalized, set italic, and surrounded by parentheses that are set as normal just like additional locants or commas.
See:Arene substitution pattern
 |  |  |
| o-Cresol | m-Cresol | p-Cresol |
The abbreviationo- (short forortho, from Ancient Greekὀρθός, meaning "upright, straight"),[8]m- (meta,μετά, (roughly) "between")[9] andp- (para,παρά, "adjoining, to the side")[10] describe the three possible positional isomers of two substituents on abenzene ring. These are usually two independent single substituents, but in case of fused ring systems,ortho-fusing is also mentioned unless the substitution pattern is regarded in the name like in[2.2]paracyclophane. In the current systematic nomenclature,o-,m- andp- are often replaced by using locants (1,2-dimethylbenzene instead ofo-xylene).
o-,m- andp- (written outortho-,meta- andpara-) are written in lowercase letters and italic.
heptane.svg) | heptane.svg) |
| 2-endo-bromo-7-syn-fluoro- bicyclo[2.2.1]heptane | 2-exo-bromo-7-syn-fluoro- bicyclo[2.2.1]heptane |
heptane.svg) | heptane.svg) |
| 2-endo-bromo-7-anti-fluoro- bicyclo[2.2.1]heptane | 2-exo-bromo-7-anti-fluoro- bicyclo[2.2.1]heptane |
exo (from Greek = outside)[11] orendo (from Greek endon = inside)[12] denotes the relative configuration of bridged bicyclic compounds. The position of a substituent in the main ring relative to the shortest bridge is decisive for the assignment ofexo orendo (according to IUPAC: the bridge with the highest locant digits[13] in the bridged ring system). The substituent to be classified is attributed with theexo descriptor when facing the bridge. It isendo configured when facing away from the bridge. If two different substituents are located on the same C atom, theexo/endo assignment is based on the substituent with higher priority according to the CIP rules.
If a bridged bicyclic system carries a substituent at the shortest bridge, theexo orendo descriptor can not be used for its assignment. Such isomers are classified by thesyn/anti notation.[13] If the substituent to be assigned points towards the ring with the highest number of segments it issyn configured (from Greeksyn = together).[14] Otherwise it is attributed with theanti descriptor (Greekanti = against).[15] If both rings possess an equal number of segments the ring with the most significant substituent according to the CIP rules is chosen.
The use ofsyn andanti to indicate the configuration of double bonds is nowadays obsolete, especially in case of aldoximes andhydrazones derived fromaldehydes. Here, the compounds were designated assyn configured when the aldehyde H and the O (of the oxime) or the N (of the hydrazone) werecis aligned. These compounds are now described by the (E)/(Z) nomenclature.Aldoximes and hydrazones classified assyn are therefore by now described as (E) configurated.[14]
When talking of diastereomers, syn and anti are used to describe groups on the same or opposite sites in zigzag projection, seeDiastereomer#Syn / anti
syn andanti are always written small and italic, locants (if used) are placed in front of the word and separated by hyphens.
The termsfac (from Latinfacies, 'external face')[16] andmer (from 'meridional')[17] can specify the arrangement of three identical ligands around the central atom in octahedral complexes. Today, this nomenclature is considered obsolete, but is still permissible.[18][19] The prefixfac describes the situation when the three identical ligands occupy the three vertices of an octahedron triangular surface. Inmer configuration the three ligands span aplane in which the central atom is located.
![fac-[CoCl3(NH3)3]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aFac-trichlorotriamminecobalt(III).png)
![mer-[CoCl3(NH3)3]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aMer-trichlorotriamminecobalt(III).png)
fac andmer are prefixed in small and italic to the complex name.
The prefixesn (normal),iso (from Ancient Greekἴσος, meaning "equal"),[20]neo (νέος, "young, new")[21] andcyclo (κύκλος, "circle")[22] are primarily used to describe the arrangement of atoms, usually of carbon atoms in carbon skeleton.n,iso andneo are no longer used in the systematic nomenclature, but still frequently in trivial names and in laboratory jargon.
The prefixn describes a straight-chain carbon skeleton without branches, whereasiso describes a branched skeleton, without specifying any further details. More generally,iso is a compound which is isomeric to then compound (a compound in which individual atoms or atomic groups are rearranged)
neo is a non-specific term for "new", usually synthetically produced substances or isomers of long-knownn compounds or natural substances (for exampleneomenthol derived frommenthol orneoabietic acid fromabietic acid). According to IUPACneo is only recommended in neopentane or the neopentyl residue.[23][24]
cyclo is a frequently used prefix for all cyclic and heterocyclic compounds. In many proper names of chemical substancescyclo is not used as a prefix but directly part of the name, for example in cyclohexane or cyclooctatetraene.
Whilen,iso andneo are written in small and italic letters, forcyclo this is only the case in inorganic compounds.[25] In organic compounds, "cyclo" is frequently used as a name component, not separated by a hyphen and also considered in alphabetical sorting.
The prefixessec andtert are used to indicate the substituent environment in a molecule. Thus, not the exact position of the substituent is described but only the substitution pattern of the adjacent atom (usually a carbon atom). Inn-butanol, the OH group is attached to aprimary carbon atom, insec-butanol to asecondary carbon and intert-butanol to atertiary carbon atom.
The termssec andtert are considered obsolete and should only be used for unsubstituted sec-butoxy,sec-butyl[26][27] or tert-butyl groups.[28][27] There are various spellings such as "sec-butyl", "s-butyl", "sBu" or "bus" which are also considered obsolete.[29][30]
The prefix "spiro" followed by aVon-Baeyer descriptor describes in the nomenclature of organic compounds ring systems linked by only one common atom, the spiro atom. If several spiro atoms are present in the molecule, the prefix "spiro" is provided with a prefix ("dispiro", "trispiro", etc.) corresponding to the number of spiro atoms. Typically "spiro" is set as normal.[31]
The termcatena (Latin: "chain") is used in the inorganic nomenclature[32] to describe linear, chain-like polymers from identical polyatomic units.[33] One example is are catenatriphosphazenes.[34][35] Related compounds in organic chemistry are thecatenanes.
The notationsn stands forstereospecific numbering, and indicates a particular way of numbering the carbon atoms in a molecule based onglycerol.
See:Cahn–Ingold–Prelog priority rules
The stereochemical descriptors (R) (from Latin rectus = right) and (S) (from lat. sinister = left)[36] are used to describe the absolute configuration of a stereocenter (usually a chiral carbon atom).[37] For this purpose, all substituents at the stereocentre are prioritized according to the CIP rules and the substituent with the lowest priority ("D") is pointed backwards (away from the viewing direction). The stereocenter is (S) configured if the remaining substituents describe a circle descending in priority ("A" → "B" → "C") to the left. The (R) configuration is assigned to the stereocenter if the direction of rotation is directed to the right.
If one molecule contains several stereocenters, a locant must be placed before the descriptor (for example, in (1R, 2S)-2-amino-1-phenylpropan-1-ol, the systematic designation ofnorephedrine). If all stereocenters are configured the same, the naming of the locants can be omitted in favor of an "all-R" or "(all-S)" spelling.
Typographically, (R) and (S) are placed in uppercase and italic; the frequently preceding locants, the enclosing round brackets and the commas, on the other hand, as normal.
-1%2c2%2c3-trichlorocyclopentane.svg)
The descriptors (r) and (s) are used to describe the absolute configuration of pseudoasymmetric centers.[38] Pseudoasymmetry occurs when four different substituents are attached to one carbon atom, two of which differ only by their absolute stereochemical configuration. Examples of such aremeso compounds such the tropane alkaloids; the parent compound istropine, whose systematic name is (1R, 3r, 5S)-8-methyl-8-azabicyclo[3.2.1]octane-3-ol. In this structure, the C3 atom—the carbon to which thehydroxyl group is attached—is pseudo-asymmetric; therefore, the stereochemical descriptor in the systematic name is written in lower-case italics rather than upper-case italics as for regular chiral atoms.
The stereodescriptorsD- (from Latin dexter, right) andL- (Latin laevus, left) are used to describe the configuration of α-amino acids and sugars.[39] First, the three-dimensional molecule must be transformed in a defined notation as a two-dimensional image ("Fischer projection").[40] For this, the C atom with the highest priority according to the normal nomenclature rules is arranged on top and the further carbon chain is arranged vertically underneath. The chiral C-atom most remote from the group with the highest priority is used for the assignment ofD- orL-. If the residue located on this carbon atom (usually an OH group) points to the left, the molecule originates from theL-series. If the residue points to the right, the descriptorD- is used.[41]
The descriptorsD- andL- are written as small capitals and separated by a hyphen from the rest of the name.[42]
Sometimes the small capitalD- andL- stereodescriptors mentioned above are mistakenly confused with the obsolete italicd- andl- stereodescriptors, which are equivalent withdextrorotatory andlevorotatoryoptical rotation, i.e. (+)- and (−)- stereodescriptors, respectively.
